Ch 1 : Acidity and Basicity
Chapter 1: Structure Determines Properties, Ch 1 contents Remember that acidity and basicity are the based on the same chemical reaction (but looking Note that organic chemists tend to think about bases by looking at the pKa's of their. Using Structure to Predict Acidity/Basicity. Strong acid the more EN the attached atom, the more acidic the molecule. C pKa = pKa = Organic Chemistry Tutorials: Acids and Bases - Molecular Structure and listed in even the most extensive tables of pKa values. . The relationship between.
This partially accounts for the driving force going from reactant to product in this reaction: What about total bond energy, the other factor in driving force? This also contributes to the driving force: Resonance effects In the previous section we focused our attention on periodic trends - the differences in acidity and basicity between groups where the exchangeable proton was bound to different elements.
The first model pair we will consider is ethanol and acetic acid, but the conclusions we reach will be equally valid for all alcohol and carboxylic acid groups.
1.24 A Summary of the Factors that Determine Acid Strength
Despite the fact that they are both oxygen acids, the pKa values of ethanol and acetic acid are strikingly different. What makes a carboxylic acid so much more acidic than an alcohol? In both species, the negative charge on the conjugate base is located on oxygen, so periodic trends cannot be invoked. For acetic acid, however, there is a key difference: This makes the ethoxide ion much less stable. Recall the important general statement that we made a little earlier: The delocalization of charge by resonance has a very powerful effect on the reactivity of organic molecules, enough to account for the difference of over 12 pKa units between ethanol and acetic acid and remember, pKa is a log expression, so we are talking about a fator of between the Ka values for the two molecules!
The resonance effect also nicely explains why a nitrogen atom is basic when it is in an amine, but not basic when it is part of an amide group. Recall that in an amide, there is significant double-bond character to the carbon-nitrogen bond, due to a minor but still important resonance contributor in which the nitrogen lone pair is part of a pi bond.
Notice that in this case, we are extending our central statement to say that electron density — in the form of a lone pair — is stabilized by resonance delocalization, even though there is not a negative charge involved.
The lone pair on an amine nitrogen, by contrast, is not so comfortable - it is not part of a delocalized pi system, and is available to form a bond with any acidic proton that might be nearby. The acid and base in a given row are conjugate to each other. The chart shows the relative strengths of conjugate acid-base pairs. These acids are completely dissociated in aqueous solution.
The conjugate bases of these acids are weaker bases than water. When one of these acids dissolves in water, their protons are completely transferred to water, the stronger base. Both hydronium ions and nonionized acid molecules are present in equilibrium in a solution of one of these acids. Their conjugate bases are stronger than the hydroxide ion, and if any conjugate base were formed, it would react with water to re-form the acid. Those bases lying between water and hydroxide ion accept protons from water, but a mixture of the hydroxide ion and the base results.
Bases that are weaker than water those that lie above water in the column of bases show no observable basic behavior in aqueous solution.
A Summary of the Factors that Determine Acid Strength - Chemistry LibreTexts
The Ionization of Weak Acids and Weak Bases Many acids and bases are weak; that is, they do not ionize fully in aqueous solution. A solution of a weak acid in water is a mixture of the nonionized acid, hydronium ion, and the conjugate base of the acid, with the nonionized acid present in the greatest concentration.
Thus, a weak acid increases the hydronium ion concentration in an aqueous solution but not as much as the same amount of a strong acid. Some potential Brnsted acids are so weak that their Ka values are smaller than water's.Organic Chemistry Acids and Bases - Reactions, Strength, Acidity, Pka & Conjugates
Ammonia, for example, has a Ka of only 1 x Although NH3 can be a Brnsted acid, because it has the potential to act as a hydrogen-ion donor, there is no evidence of this acidity when it dissolves in water. The Leveling Effect of Water All strong acids and bases seem to have the same strength when dissolved in water, regardless of the value of Ka.
This phenomenon is known as the leveling effect of water the tendency of water to limit the strength of strong acids and bases.
A similar phenomenon occurs in solutions of strong bases. Strong bases react quantitatively with water to form the OH- ion.